1. Field of the Invention
This invention relates to a slurry (polishing medium) especially suited for the polishing of silicon substrates having thin films, and to a semiconductor manufacturing process making use of the same.
2. Description of the Related Art
As the integration of semiconductor devices is being made higher, device structure is being made three-dimensional by forming their wiring in multilayer or making capacitor cells three-dimensional in memory LSIs. However, making it three-dimensional by multilayer formation may consequently provide device surfaces with a step (difference in height at surface). This step may cause break of wiring patterns, or marginal shortage of the depth of focus in lithographic processes. To prevent this, techniques for the planarization of multi-layered films are indispensable. To make planarization in regions of a millimeter order, chemical-mechanical polishing (abbreviated “CMP”) is commonly used. This chemical-mechanical polishing is also utilized in shallow-trench isolation, metal plug and wire formation (damascene), and its studies are made on silicon oxide films, tungsten, polysilicon, copper and so forth.
However, when this chemical-mechanical polishing is used, any dishing occurring at the time of polishing and any step caused by erosion may additionally come into question. Incidentally, the dishing is a phenomenon that, when, e.g., buried wiring of a metal is formed in an insulating film, a hollow like a dish is produced as a result of the polishing of the metal in excess beyond the insulating film. The erosion is a phenomenon that, in an area where wirings stand close together, a step is produced as a result of the polishing of wiring and insulating film in excess to the part having no wiring. When the chemical-mechanical polishing is used, it is required to keep these phenomena from occurring.
Accordingly, when a buried pattern is formed in a substrate, it is common to provide a barrier layer between the substrate and the buried pattern and polish the buried material and the barrier material through two-stage polishing making use of different slurries so that the phenomena causative of such surface steps can be avoided. More specifically, dales are formed in a substrate, and on its surface a barrier film comprised of a barrier material and a buried film comprised of a buried-film material are formed in order, followed by the two-stage polishing making use of different slurries to obtain a buried pattern buried in the dales while keeping the dishing and erosion from occurring.
In this method, a slurry having a much higher polishing rate on the buried-film material than the polishing rate on the barrier material is used in the first-stage polishing to remove the buried film from the surface of the barrier film at its part other than the dales. Then, in the second-stage polishing, a slurry having a much higher polishing rate on the barrier material than the polishing rate on the substrate material is used to remove the barrier film from its part other than the dales. As methods for such polishing, the following techniques (a) to (c) are known, for example.    (a) Japanese Patent Application Laid-open No. 10-163142 discloses that, in the chemical-mechanical polishing of tungsten (W)/silicon oxide (SiO2) film, the dishing can be made less occur by using a polishing composition having a polishing-rate ratio (selection ration) of W/SiO2≧6 and also having a great polishing rate on tungsten.    (b) Japanese Patent Application Laid-open No. 10-214834 discloses a polishing method in which a silicon oxide film, a titanium (Ti) film, a titanium nitride (TiN) film and a tungsten film are formed on a substrate in this order and thereafter a pattern of tungsten contact holes is formed by chemical-mechanical polishing. In this method, first polishing is performed until the titanium film comes uncovered, using a slurry having a polishing-rate selection ratio of W>Ti (preferably W/Ti>2) and W/SiO2>3, and thereafter second polishing is performed using a slurry having a polishing-rate selection ratio of Ti>SiO2 and 0.5≦W/SiO2≦3, to keep the erosion from occurring.    (c) Japanese Patent Application Laid-open No. 2001-44156 discloses a polishing method in which, when a silicon oxide film, a tantalum nitride (TaN) film and a copper (Cu) film are formed on a substrate in this order and a pattern of wiring or the like is formed by chemical-mechanical polishing, first polishing is performed by polishing Cu and TaN until the SiO2 film comes uncovered, and thereafter second polishing is performed using a slurry having a polishing-rate selection ratio of SiO2/Cu=0.2 to 5, to keep the erosion from occurring.
Where the above method (a) is used when buried wiring is formed in a silicon substrate, the selection ratio of polishing rate is so great that the polishing end-point detection at the time the SiO2 film has come uncovered can be made with ease. Thus, according to this method, overpolishing may less occur, so that the dishing may less occur, as so reported. In fact, however, tungsten at the part of wiring is slightly polished away to cause the dishing correspondingly. This slight dishing may be not on a level problematic in present generation, but is considered to come problematic in future generation which may make progress for finer patterning. Also, while the SiO2 is little polished in an area having no tungsten pattern, the tungsten at the part of wiring in an area where the tungsten pattern stands close together is polished and at the same time the SiO2 is polished on, though slightly, to cause the erosion inevitably. There is such a problem, too.
In the method (b), the selection ratio in the second polishing is set to be 0.5≦W/SiO2≦3. However, experiments made by the present inventors have come upon a case in which dishing and erosion have occurred in fact when the polishing is performed using a slurry having the polishing-rate ratio of W/SiO2 of about 0.5 or 3.
As also disclosed in the above Japanese Patent Application Laid-open No. 10-214834, a slurry making use of silica particles as abrasive grains and ammonium hydroxide or potassium hydroxide as a pH adjuster is used in the second polishing. This slurry is a slurry commonly used for the polishing of insulating films, and it is noted that “tungsten is little polished” (page 8, line 30). In fact, however, tungsten is slightly etched by using such an alkali type slurry. Hence, where CMP is performed especially on fine wiring or plugs of tungsten, the slurry may enter through seams formed when tungsten film is formed, to cause etching in the wiring or plugs, so that key holes may be produced to cause an increase in wiring resistance. Such a problem may arise.
In addition, since the tungsten is little polished by using this slurry, reverse dishing (a state in which tungsten plugs or wiring comes protruded from the insulating films) may occur to produce an additional step. Moreover, where any tungsten film has remained in the first polishing, the part of tungsten having remained is not polished in the second polishing to provide a short. Also, this slurry may separately materialize “average polishing rates of titanium film and oxide film are both 100 nm/minute” (page 8, line 29) and “a polishing-rate selection ratio that the polishing rate on the upper-layer conductive film (W) is three times or less the polishing rate on the insulating films” (page 10, line 34). However, it is impossible to materialize both of these simultaneously because of the use of the slurry with which the tungsten is little polished. Furthermore, titanium and tungsten are polished without use of any oxidizing agent, and hence, in fine patterns, any titanium and tungsten remaining as polish leavings may again adhere to cause a short inevitably.
In this method, it is also necessary to clean wafers sufficiently after the first polishing, because the slurry used in the first polishing is acidic and the slurry used in the second polishing is alkaline. If the cleaning is insufficient, the slurry may contaminate in the second polishing to cause a change in action of chemical components contained therein and at the same time abrasive grains alumina used in the first slurry may agglomerate under alkaline conditions to cause polish mars.
According to the above method (c), the selection ratio SiO2/W in the second polishing is set to be 0.2 to 5. However, experiments made by the present inventors have come upon a case in which dishing and erosion have occurred when the damascene to form tungsten wiring is performed using a slurry having the polishing-rate ratio of SiO2/Cu of about 0.2 or 5. Also, in this method, a slurry containing alumina abrasive grains is used as a copper-polishing slurry and a slurry containing silica abrasive grains is used as a SiO2-polishing slurry. However, in general, under acidic conditions, alumina stands charged positively and on the other hand the silica stands charged negatively. Hence, where these are mixed, alumina and silica attract each other to agglomerate to cause polish mars.